A vehicle has parking sensors to detect an obstacle behind the vehicle. The parking sensors determine a distance of the vehicle from the obstacle using ultrasonic signals when backing a vehicle. The parking sensor operates at ultrasonic frequencies. The parking sensor outputs an ultrasonic detecting signal to detect whether any obstacle is behind the rear of the vehicle and receives an ultrasonic signal as reply from the obstacle. A vehicle generally requires multiple parking sensors to cover the entire rear of the vehicle which makes it a cost intensive solution. Also, the ultrasonic parking sensors use a time division obstacle detecting method in which each sensor sends and receives ultrasonic detect signal in a defined time slot. Thus, the process of detecting obstacles using ultrasonic sensors is time consuming which is unsafe in vehicles moving with high velocity.
Ultrasonic parking sensors require the measurement and drilling of holes in the vehicle's bumper to install transducers. There are risks associated with drilling and mounting the transducers into the bumper. The performance of the Ultrasonic sensors is sensitive to temperature and atmospheric conditions such as snow and rain. The performance of ultrasonic sensors is severely degraded when the sensors are covered with snow. In addition, the range over which the ultrasonic sensors operates is limited.
The use of radars in automotive applications is evolving rapidly. Radars do not have the drawbacks discussed above in the context of ultrasonic sensors. Radar finds use in number of applications associated with a vehicle such as collision warning, blind spot warning, lane change assist, parking assist and rear collision warning. Pulse radar and FMCW (Frequency Modulation Continuous Wave) radar are predominately used in such applications. In the pulse radar, a signal in the shape of a pulse is transmitted from the radar at fixed intervals. The transmitted pulse is scattered by the obstacle. The scattered pulse is received by the radar and the time difference between the start of transmission of the pulse and the start of reception of the scattered pulse is proportional to a distance of the radar from the target. For better range resolution, a narrower pulse is used which requires a high sampling rate in an ADC (analog to digital converter) used in the pulse radar. In addition, sensitivity of a pulse radar is directly proportional to the power which complicates the design process of the pulse radar.
In an FMCW radar, a transmit signal is frequency modulated to generate a ramp segment. An obstacle scatters the ramp segment to generate a received signal. The received signal is received by the FMCW radar. A signal obtained by mixing the ramp segment and the received signal is termed as an IF (intermediate frequency) signal. The frequency of the IF signal is proportional to the distance of the obstacle from the FMCW radar. The IF signal is sampled by an analog to digital converter (ADC). A sampling rate of the ADC is proportional to the maximum frequency of the IF signal and the maximum frequency of the IF signal is proportional to the range of a farthest obstacle which can be detected by the FMCW radar.
The range is the distance of the obstacle from the FMCW radar. Thus, unlike in the pulse radar, the sampling rate of the ADC in the FMCW radar is independent of the range resolution. Typically in an FMCW radar, multiple identical ramp segments are transmitted in a unit called as frame. Range resolution defines the capability of the FMCW radar to resolve closely spaced objects. The range resolution is directly proportional to a bandwidth of the transmitted ramp segment. Also, the transmitted ramp is required to meet the phase noise specifications that are needed for achieving the desired performance levels. However, it is difficult, because of hardware limitations, for a local oscillator in the FMCW radar to generate a ramp segment with a wide bandwidth and simultaneously meeting the phase noise specifications. Thus, it is important for the FMCW radar to transmit a wide bandwidth ramp segment for high range resolution and at the same time maintaining optimum performance level and accuracy.